Storage extents in a storage system are provided to a host computer as a logical set called data volumes. The host computer recognizes the data volumes as one logical device. Conventionally, the data volumes are created from one large set (for example, a RAID group) of physical storage devices (for example, magnetic hard disk drives). Therefore, storage extents with homogenous physical characteristics (I/O performance and cost) are allocated to the data volumes.
However, accesses to the data volumes may not be homogenous in all areas in the data volumes. For example, when a server virtualization technique is used to construct an information system, a plurality of virtual disks with significantly different access frequencies due to the roles or applications of the virtual machines may be stored in the same data volume.
As a result, data with low access frequency is continuously placed on an expensive data volume, or data with high access frequency is continuously placed on a low-speed data volume. Therefore, the cost performance of the entire storage system is reduced.
Patent Literature 1 described below discloses a technique for improving the cost performance of the storage system. In Patent Literature 1, a dynamic chunk allocation function is used in which the data volumes are divided into sets of data blocks called chunks, and the chunks constituted by physical data blocks with different performances and costs are dynamically allocated to the data volumes. For example, the cost performance of the storage apparatus can be optimized by allocating a high-speed, expensive chunk to a storage extent with high access frequency to the data volume and allocating a low-speed, inexpensive chunk to a storage extent with low access frequency.
The storage system of this type includes dynamic chunk allocation volumes (DCAVs) in which physical blocks that record data of the data volumes are dynamically changed. The dynamic chunk allocation function is realized by the following two processes.
A first process is carried out when a write command is received from a host for a segment that is not allocated with a physical block in the DCAV. In the present process, if a chunk is not yet allocated to the segment designated by the write command, a chunk constituted by physical blocks is allocated to the segment.
A second process is a process of changing the physical blocks for arranging the data chunk-by-chunk according to a write command frequency and a read command frequency received by the data volumes or according to a policy set by management software. As a result of the second process, data recorded in the volumes can be migrated from a physical storage device to another physical storage device with different characteristics chunk-by-chunk without changing the logical configuration of the data volumes as seen from the host computer. The data migration can change the data volumes to a physical configuration with a higher cost performance in accordance with the use frequency. The data on an old physical storage device can be migrated to a new physical storage.
The performance and the number of chunks allocated to the data volumes are determined depending on the accesses from the host computer, and the performance and the number cannot be deterministically predicted. Therefore, pools (chunk pools) for storing the chunks in advance are necessary. An example of creating the chunk pools includes a method of using a set of data volumes (pool volumes) managed by dividing the data volumes into chunks.
Meanwhile, Patent Literature 2 describes an external storage connection function in which data volumes in another storage apparatus are virtualized as if such data volumes belong to the storage apparatus, thereby providing access to the host computer. As a result of the function, for example, introducing a new storage apparatus with the external storage connection function in an existing storage system allows using a function provided by the new storage apparatus without carrying out a configuration change or a data rearrangement of the existing storage apparatus.
Patent Literature 3 described below describes a technique of reducing the load related to a process for an external device in a storage system with the external storage connection function.